Obesity is a leading cause of morbidity and mortality worldwide that arises from disorders affecting energy homeostasis, the process whereby cumulative energy intake is matched to energy expenditure over time. This process is dependent on hormones such as insulin and leptin that circulate at concentrations proportionate to body fat stores and act in the hypothalamus to reduce food intake and body weight. Defective hypothalamic signaling by these hormones is implicated in obesity pathogenesis and poses an important obstacle to its treatment. Thus, a key priority for the field is to delineate more clearly the mechanisms underlying normal and abnormal insulin and leptin signaling in the brain. Recent studies have identified a subset of neurons in the hypothalamic arcuate nucleus (ARC) that utilize the insulin receptor substrate-phosphotidylinositide-3 kinase (IRS-PI(3)K) pathway to respond to both insulin and leptin. Since our preliminary data indicates that PI(3)K signaling is required for leptin's suppressive effects on food intake, we propose to 1) determine which behavioral, hypothalamic and metabolic effects of leptin require PI(3)K signaling; 2) identify the neuropeptide phenotype of ARC neurons that respond to insulin and leptin by activating the IRS-PI(3)K pathway; 3) determine if increased PI(3)K signaling in ARC neurons mimics the hypothalamic effects of insulin and leptin, and if impaired PI(3)K signaling in this brain area causes obesity; and 4) determine if resistance to leptin's anorexic effect that develops in diet-induced obesity is paralleled by impaired activation of PI(3)K signaling, and if it can be reversed by increasing PI(3)K activity in the ARC. To achieve these objectives, we will apply state-of-the-art pharmacological, biochemical, histochemical and adenoviral strategies to study the role of IRS-PI(3)K signaling in ARC neurons in rodent models of normal and abnormal energy homeostasis in vivo. This information will help to clarify how energy homeostasis occurs and has the potential to identify new targets for drug development in obesity treatment.